How can I detect NULL within Active X

Hello Folks:

I am (via DTS) copying a table from an ODBC Source (Notes yuc) into SQL.  My SQL based destination table has several columns of numeric data with the "NOT NULL" constraint on.  My probelm is, the source is not so constrained.  My goal, when a NULL value is encountered, is to replace it with 0.  To that end, I have written the below transform.  I always get an "object required" error, or a type mismatch.  (I have tried many permutations with IS NULL, = NULL, ISNULL, etc.)  The fundamental question I have for you all:  How can I tell that s DTS Source Value is NULL?  It is probably simple, but I am missing it.


'**********************************************************************
'  Visual Basic Transformation Script
'************************************************************************

'  Copy each source column to the destination column
Function Main()
      DTSDestination("ReforcastDecemberIT") = SourceOr0("rD998")
      DTSDestination("ReforcastNovemberIT") = SourceOr0("rN998")
    ' many more instructions.  You get the idea.
      DTSDestination("EstimateMarchContractors") = SourceOr0("M997")
      DTSDestination("EstimateFebruaryContractors") = SourceOr0("F997")
      DTSDestination("EstimateJanuaryContractors") = SourceOr0("J997")
      Main = DTSTransformStat_OK
End Function


Function SourceOr0(SourceColumn)
      if DTSSource(SourceColumn) = NULL then
         SourceOr0 = 0
      else
         SourceOr0 = DTSSource(SourceColumn)
      endif
      
end Function